Render only existing children in staff view of split_test.

This fixes BLD-1224.

Root cause is that group_id_to_child contains deleted groups.
Other solution is to clean group_id_to_child of split_test when group was deleted from group configuration page. But this will affect publishing. So this solution was implemented but skipped.

@tymofij @cahrens please review.
@cahrens why we run here over group_id_to_child? I think it is better run over self.children and all children will exist. I do not think we should rely here on group_id_to_child at all.